I'd second what Joost said.  As an example of a platform specific approach, you could do something like this on systems with a /proc file system (the example is for Linux, but it should be quite similar elsewhere):

#!/usr/bin/perl

my $ppid = getppid();   # parent PID

my $procfile = "/proc/$ppid/cmdline";

open my $fh, "<", $procfile or die "Cannot open $procfile: $!";
my $cmdline = <$fh>;
$cmdline =~ tr/\0/ /;
close $fh;

print  "cmdline: $cmdline\n";
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As already said, this would require that the parent process is still running.